The major goal of Project I is to assess the genetic and environmental etiologies of reading deficits (including word recognition skills, reading fluency, and reading comprehension), ADHD, and their comorbidity, as well as their covariation with measures of different language skills, mathematics, processing speed, and executive functions. To accomplish these objectives, an extensive psychometric test battery will be administered to a sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair has significant reading or writing difficulty (WD), to an independent sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair meets symptom criteria for ADHD, to a comparison sample of 50 pairs of identical twins and 50 pairs of fraternal twins with no school history of reading or writing deficits or ADHD, and to the siblings of all twin pairs with a history of RD, WD, or ADHD. In collaboration with investigators from Research Projects ll-IV, resulting data will be used to assess the genetic and environmental etiologies of reading deficits, ADHD, and their comorbidity, as well as their covariation with measures of other psychopathology, reading, language and perceptual processes, and executive functions. RELEVANCE (See instmctions): Reading disorder and ADHD are common disorders that lead to significant impairment forthe individual and public health costs for the larger society. By identifying specific risk and protective factors, etiologically- informative studies may eventually lead to the development of effective methods of prevention or eariy intervention.